The aim of this study is to learn more about how age-related changes in the cardiovascular system affect the ability of humans to exercise with large and small muscle groups. Currently it is unclear if the reduced exercise capacity seen in humans as they age is a result of reduced maximal muscle blood flow or a reduction in cardiac output. In this study, the investigators will determine whether there is evidence to support either possibility. We made strong progress in the non-invasive aspects of this protocol and conducted studies that allowed us to determine the relationship between skeletal muscle mass and exercise tolerance in aging humans. We have also looked at the relationship between cardiac output, muscle mass, and the rise in oxygen uptake that occurs with exercise in aging humans. Additionally, we have initiated our invasive studies to better define the leg blood flow responses to exercise in aging humans. Data from this project has been used as a pilot study that has permitted Dr. David Proctor to submit a grant proposal to the NIH. The approaches used to stud exercise responses and use of exercise protocols in aging subjects have been expanded at Mayo as a result of this protocol.